'Microsoft is providing the full .NET server stack in open source, including ASP.NET, the compiler, the .NET Core Runtime, Framework and libraries, enbaling devvelopers to build with .NET across Windows, Mac or Linux'.

This is old story, since 2004 the 'mono project' was a open source implementation of .NET platform.

Now coming to OWIN, which is short for Open Web Interface for .NET OWIN) defines a standard interface between .NET web servers and web applications.

You can find everything you want to know about OWIN here at this master site.